1. Field of the Invention
The present invention relates to a production method of a toner for developing an electrostatic latent image used for development of an electrostatic image in an electrophotography, an electrostatic recording method, an electrostatic printing process or the like (hereinafter, it may be simply referred to as “a toner”). Particularly, the present invention relates to a production method of a toner for developing an electrostatic image which is excellent in productivity in production and shelf stability at high temperature, and produces no odor when printing using the same.
2. Description of the Related Art
Recently, the need of colorization of printed images for image-forming devices employing the electrophotography method such as copying machines, facsimiles, printers or the like is increasing. Since in color printing, a precise image requiring reproduction of a clear color tone such as a photograph or the like is also printed, high resolution is necessary. Accordingly, a colored toner which can suffice such requirements is demanded.
Attaining both excellent transferability and dot reproducibility thereby increases resolution of an image so as to obtain excellent printing performance. To obtain such a printing performance, a spherical toner with a small particle diameter is suitable. As a method of producing such a toner, a polymerization method is proposed. In a pulverization method, which is a conventional method to produce a toner, a shape of the toner thus produced is irregular. In addition, production of a toner with a small particle diameter involves a decrease in yield and large energy consumption for pulverization. To the contrary, in a polymerization method, yield is high and energy consumption is low since a pulverization process is not required. Further, a spherical toner can be easily produced.
As a method of producing a toner by the polymerization method, there may be a suspension polymerization method, an emulsion agglomeration polymerization method, a dispersion polymerization method, and so on. In the suspension polymerization method, firstly, a polymerizable monomer, a colorant, and if required, other additives are mixed to prepare a polymerizable monomer composition, and the polymerizable monomer composition is dispersed in an aqueous dispersion medium containing a dispersion stabilizer. Next, high shear is applied to the aqueous dispersion medium having the polymerizable monomer composition dispersed by means of a high-speed agitator or the like to form droplets of the polymerizable monomer composition. Then, the aqueous dispersion medium having the droplets of the polymerizable monomer composition dispersed is polymerized in the presence of a polymerization initiator followed by removal of the dispersion stabilizer, washing, filtering, dehydrating and drying, thus, colored resin particles are obtained. Further, the colored resin particles are mixed with an external additive such as an inorganic fine particle or the like to obtain a polymerized toner. Further, if required, the polymerized toner is mixed with a carrier to obtain a two-component developer.
As aforementioned, compared to the conventional pulverization method, obtaining the colored resin particles by the polymerization method has big advantages that spherical colored resin particles with a small particle diameter can be formed in the stage of forming particles (in the polymerization method, in a stage of forming droplets and polymerizing) and a particle size distribution can be controlled to be narrower. However, with the recent increasing demand for image printing with high resolution and high image quality, it is attempted to decrease the diameter of toner particles. In addition, a new problem is found in the polymerized toner.
The problem is described as follows: in a polymerization process of a production method of a toner, besides desired colored resin particles, undesired particles with a very small particle diameter are produced as a by-product. They affect production efficiency and printing performance of the toner.
As the by-product microparticles, there may be mainly microparticles with a diameter of less than 0.6 μm (or with a so-called submicron order particle diameter) and containing no colorant (hereinafter, such microparticles are referred to as “small diameter microparticles”).
If such small diameter microparticles are produced as a by-product, a part of the microparticles released clogs a filter upon filtration of the obtained colored resin particles from an aqueous dispersion medium. A filtration rate thereby decreases so as to reduce the production efficiency of a toner.
Moreover, if a polymerized toner containing a lot of small diameter microparticles is used for image forming, since the small diameter microparticles have high adherence, they are likely to adhere to members in a developing system. Consequently, the attached small diameter microparticles are gradually accumulated so as to cause filming (adherence) to the members when plural prints are printed using the polymerized toner. For example, when the microparticles cause filming on a photosensitive member in the developing system, a surface of the photosensitive member is poorly charged and a desired electrostatic latent image cannot be formed on the photosensitive member. As a result, problems such as generation of a fog on a recording medium and so on are caused, which result in that no excellent image is obtained and printing performance of the toner including printing durability and so on may be reduced.
According to the polymerization method, for example, colored resin particles with a volume average particle diameter “Dv” of from about 3 to 15 μm can be readily formed. However, separation of the colored resin particles with a desired particle diameter and undesired small diameter microparticles becomes difficult since a targeted particle diameter range comes closer to the above-mentioned particle diameter of the small diameter microparticles as it shifts to a small particle diameter side. Thus, development of a production method which is capable of inhibiting production of by-product small diameter microparticles, and excellent in printing performance and production efficiency of a toner is desired.
In order to meet such demands, various approaches and insights are provided to a method of inhibiting the production of by-product small diameter microparticles.
For example, PCT International Publication Number WO 2006/013847 discloses a production method of a polymerized toner characterized in that: a polymerizable monomer composition containing a polymerizable monomer, a colorant and a charge control agent is charged into an aqueous dispersion medium and stirred; t-butylperoxy-2-ethylhexanoate (product name: PERBUTYL O; manufactured by: NOF Corporation) is added thereto as a polymerization initiator to form droplets; and a hydroquinone compound is added thereto as a water-soluble polymerization inhibitor (an inhibitor of small diameter microparticle production) before polymerization.
However, as a result of consideration by the inventors of the present invention, it is found that the hydroquinone compound used as an inhibitor of small diameter microparticle production in WO 2006/013847 is not fully effective in inhibiting the production of small diameter microparticles as a by-product.
Accordingly, an inhibitor of small diameter microparticle production which is more effective than the hydroquinone compound in inhibiting the production of by-product small diameter microparticles is desired.